1. Field of the Invention
This invention relates to electrical control systems and more particularly to a control system in which a plurality of geographically distributed command stations are interconnected by a single pair of conductors. The command stations drive power control delays in response to polarity reversal of the line pair. Each of the command stations is capable of controlling the polarity status of the line pair, of driving a relay for controlling power to an electrical load, and may include means for monitoring the polarity status of the line pair.
2. Prior Art
Various systems suitable for controlling power to an electrical load from one or more remote command stations are known, as exemplified by the following patents:
______________________________________ U.S. Pat. No. Patentee Name Issue Date ______________________________________ 2,788,517 W. L. Smoot et al April 9, 1957 4,017,832 D. E. Gilbert April 12, 1977 4,136,333 Sumida et al January 23, 1979 4,131,839 B. R. Springer December 26, 1978 4,174,064 Pratt, Jr. November 13, 1979 4,086,568 Watts, Jr. et al April 25, 1978 2,107,902 J. H. Oliver February 8, 1938 ______________________________________
The Smoot system differs from the present invention in that each of the remote control units of Smoot as well as the power relay circuit require separate power supplies. If the power supplies consist of batteries, a problem of reliability is created in that one or more of the control or power relay units may be disabled unexpectedly if its particular battery is exhausted. If, instead, each of the units is connected to an alternating current outlet for reliable power, the expense and complexity of the system is vastly increased since a separate transformer circuit will be necessary for each unit in the system and wiring will have to be installed between each of the control units and an A.C. outlet. This substantially decreases the usefulness of the remote control system since the control unit will have to be located within a reasonable distance from available power outlets.
The system disclosed by Gilbert appears to be limited to a single command station connected to a single power control relay unit. No provision is made for installing an arbitrary number of control and monitoring units to suit the particular requirements of a given installation. Since the proper operation of the circuit is admittedly dependent on overall electrical circuit characteristics, it would further appear that modification of the circuits so as to accommodate different numbers of power control relay units or command units would be impractical. In addition, in cases where long runs of transmission lines are required between the control/monitoring unit and the power relay unit, the inherent resistance of such transmission line would affect the operating characteristics of the system.
Sumida et al discloses a control system in which multiplexed signals are transmitted through a single line pair to a number of relay control circuits. This system differs from the present invention in that it aims to provide independent control over a number of electrical devices. Thus, the control circuits are considerably more complex than those of the present invention and rely on digital techniques. The remaining patents cited above differ in various significant respects from the present invention.
A continuing need exists for a reliable, low cost, power control system which can be easily installed without need for costly electrical conduits and is flexible enough to meet the requirements of a large variety of residential as well as industrial applications.